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Successful biological nutrient removal often requires a supplementary carbon source

Beef processing facility

A beef slaughter and processing plant recently completed an upgrade to its 0.5 MGD wastewater treatment facility to comply with its total maximum daily load limits. The upgrades included a dual train, four stage Bardenpho process.

Prior to the upgrade, the facility historically discharged >200 mg/L TN. Effective October 2013, its new permit required an annual TN average concentration of <8 mg/L and a daily maximum TN of 130 mg/L at a flow rate of 0.5 MGD.

With very high nitrogen loading, peaks of up to 300 mg/L TN , and with low carbon/nitrogen ratios in the anoxic zones, the plant was designed to use supplemental carbon to facilitate denitrification to comply with the National Pollutant Discharge Elimination System permit limits. MicroC 2000 was chosen as the carbon source due to safety, low cost and high quality.

During commissioning, the facility experienced a number of challenges achieving denitrification, including incomplete denitrification with nitrite accumulation, debilitating thick brown foam/scum, and process instability and unequal performance in the parallel BNR treatment trains.

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Environmental Operating Solutions Inc. (EOSi) conducted a detailed review of the plant operating data to focus on developing a cost-effective solution. The first step in the assessment was microbial analysis to determine the nature of the thick brown foam. It showed the presence of Thiothrix spp. in several foam samples. Further investigation indicated that the high COD utilization rate in the post anoxic reactor, combined with short hydraulic retention time and low concentration of available nitrogen for assimilation, resulted in excess brown foam and sludge bulking.

Data collection and observation of system response with static modeling and simulation analysis showed the presence of high dissolved oxygen (DO) in the anoxic zones, caused by excessive aeration from an oversized blower with limited turndown control. DO was also being entrained within a manhole splitter box that divides flow from the anaerobic zone into the two pre-anoxic zones.

EOSi and plant personnel were able to develop an operating strategy which incorporated changes to achieve a more steady biological process and to reduce effluent TN from >200 mg N/L to less than 3 mg N/L. This brought the plant into compliance. Key operational changes included:

  • Replacing the blower with one having better turndown control.

Piping changes in the influent manholes to reduce DO entrainment.

Revised carbon feed strategy as recommended by EOSi.

Within weeks of implementing the process enhancements, DO concentrations were optimized throughout the process. Thick brown foam was virtually eliminated throughout the entire plant. The plant achieved full denitrification with an effluent of NOx (Nitrate+ Nitrite) <0.2 mg/L. Beyond achieving consistent compliance, the plant also was able to stabilize solids retention time (SRT) and clarifier performance.

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